1. Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices, and more specifically to a method used to form a strained silicon layer to accommodate a channel region for a metal oxide semiconductor field effect transistor (MOSFET) devices.
2. Description of Prior Art
A promising method for improving MOSFET device performance is enhancement of carrier mobility. Both hole and electron mobility enhancement can be accomplished via the employment of surface channel, tensile strained silicon layers. Strained silicon layers used for MOSFET channel regions have been formed via epitaxial growth of silicon on relaxed silicon—germanium (SiGe) pseudo substrates. Good film quality of strained silicon layers, and of relaxed SiGe layers, are important factors for enhanced performance of strained silicon MOSFET devices, however substrate layers exhibiting high defect density, in terms of dislocations, stacking faults, twins, etc., will reduce carrier mobility.
A high quality relaxed SiGe layer, to be subsequently overlaid by a strained silicon layer, can be produced via the use of a thick SiGe buffer layer deposited on an underlying semiconductor substrate. However, there are several disadvantages to the growth of a thick SiGe buffer layer. First, a thick SiGe buffer layer at a thickness greater than a micrometer negatively impacts throughput adding unwanted processing costs. Secondly, the defect density of thick SiGe layers can be as high as 1E7 defects/cm2. The use of a this SiGe buffer layer on a silicon on insulator (SOI) substrate, does allow defect densities to be decreased, however still in the unacceptable range of about 1E6 defects/cm2.
This invention will feature a method of obtaining a desired, strained silicon layer on a relaxed, low defect density semiconductor alloy layer, and an alloy layer such as a SiGe layer. This is accomplished via the growth of the strained silicon layer on an underlying composite layer, which in turn is comprised of a silicon layer sandwiched between two strained layers, such as SiGe layers. As the thickness of the top SiGe strained layer of the composite layer increases via epitaxial growth, dislocations form in the sandwiched, initially unstrained silicon layer, allowing the top, or overlying initially strained alloy layer to relax, and exhibit a low defect density. The desired strained silicon layer is now epitaxially grown on the underlying, relaxed semiconductor alloy layer. A key advantage of this invention is the low dislocation density in the relaxed SiGe layer, underlying the strained silicon layer. Prior art such as Chu, et al., in U.S. Pat. No. 6,059,895, as well as Ek, et al., in U.S. Pat. No. 5,759,898, describe methods of forming tensile strained silicon layers via SiGe relaxed layers, however these prior arts do not describe the novel process sequence disclosed in this present invention in which a strained silicon layer is formed on a relaxed, low defect density, semiconductor alloy layer.